In a fixation system with a heating roll which is commonly employed for electrophotographic apparatus, a fixation roll having a surface parting layer consisting of a fluorocarbon resin is employed and the copying paper carrying the image formed with an unfixed toner powder is fed between the surfaces of the fixation roll and press rolls to fix the toner on the copying paper by fusing it using heat and pressure.
For the heating roll fixation method, the heat pipe-type fixation roll with multiple heat pipes embedded in the thick wall of the core in the direction parallel to its axis has been developed, to prevent any uneven temperature distribution on the surface of the fixation roll immediately after energizing the halogen heaters and during service as a result of heat loss due to the passage of copying paper.
The inventor and others have previously proposed a type of fixation roll exhibiting a more uniform surface temperature distribution than that of conventional fixation rolls by improving the structure of the heat pipes embedded in the heat pipe type fixation roll. (Unexamined Japanese Patent Publication No. 4-139481.) The heat pipe employed in the fixation roll consists of a metal tube that encapsulates an operating fluid and a wick of elastic metal wires bent to form a hairpin shape in such a manner that the mutual distance gradually increases from the middle of the tube toward the free end, and the wick in the special shape causes the operating fluid to travel during reflux in the axial direction with the rotation of the fixation roll to substantially improve the surface temperature distribution of the fixation roll.
In the process of subsequent tests and reviews with the aim of further improving the temperature rise speed of the heat pipe type roll and the surface temperature distribution during the service of the fixation roll, the inventor and others discovered that the degree of fitting tightness of the heat pipe to the wall of the core when embedding the heat pipe in the fixation roll has an influence on the improvement of the characteristics.
For embedding the heat pipe in the fixation roll, such methods as first tightly fitting the heat pipe with the inner wall surface of the bore in the core and then sealing each end of the cylindrical bore with aluminum or an equivalent component to secure dimensional precision for the tight fitting of the heat pipe with the matching bore provided in the axial direction of the core as well as provisionally heating the core and shrink fitting the heat pipe are exercised. These methods require stringent control of the dimensional precision of the diameters of the bore and the heat pipe to obtain a firm fitting.
As a result of studies, the inventors and others have discovered that by encapsulated water as the operating fluid in the heat pipe and subsequently heating the water to allow it to evaporate after inserting the heat pipe into the core so as to cause the heat pipe to undergo plastic deformation and expand due to the pressure and to tightly fit against the inner wall of the bore in the core, resulting in an integral and monolithic structure, the heat pipe fits firmly onto the inner wall of the bore and thus the performance of the fixation roll can be further improved as compared with the abovementioned method without the need for stringent control of the dimensional precision of the diameters of the bore and the heat pipe.